Mercedes-Benz Forum banner

Reinstall of fuel injection pump?

2K views 10 replies 9 participants last post by  Chas Stricker 
#1 ·
Question for the group... Last week I brought the fuel injection pump off of my OM366A to the good folks at Merex in Germany. With zero notice, they were able to take it to Bosch and have it reset to the 170 HP settings for me. I have it back with me now to reinstall on my U1300L. Are there any special considerations before I just bolt the unit back on (timing markers lined up, of course) and try to start it up? Do I need to pre-prime it in any way?
 
#2 ·
You will need to prime it and then bleed the lines to the injectors otherwise you will be chasing bubble. There have been some recent posts on this topic, might be a good time for an electric lift pump from the tank.
 
#4 ·
#5 ·
Yup, I can confirm, assume there will be lots of air - expect to end up with lots of diesel spillage as you crack open lines to get rid of bubbles. Kitty litter helps to soak it up.
The lift pump is not very good at pushing air out without cracking open the line right at the IP. My IP is of the older type with a bleed screw. Yours(later type) might not have the bleedscrew and you will have to loosen the IP inlet banjo to get all the air out.
 
#6 ·
When I did mine, I used a low pressure pump in combination with hand priming on the lift pump. Once you hear fuel returning back to the tank the IP is primed. I didn't do anything to the injector lines and it only took about 15 seconds of cranking to get it to fire. The important part is setting the spill port timing properly. The procedure in the workshop manual is pretty good, as long as you have the timing marks on the crank pulley and the ones on the timing gear case aligned before you start, the procedure works. It is a bit messy, but once you get the timing done right, it should start pretty easy.
 
#7 ·
In case you don't have the WSM handy, bleeding the fuel system contains 3 sequential steps. These include bleeding the fuel filters, bleeding the IP, and lastly bleeding the injector lines. As mentioned, if you get a good bleed on the filters and the IP, the injector lines should purge themselves.

Bleeding Fuel Filters:
Should be two bleeder screws on top of the fuel filter housing that should be cracked open 1-2 turns. Then actuate hand pump until all bubbles are gone. Tighten both bleeder screws.

Bleeding IP:
Pretty much the same as bleeding the fuel filters except there is only one bleeder screw (maybe, maybe not as Peter mentioned previously) near the fuel return line at the right hand side of the IP. Crack open 1-2 turns and actuate hand pump until bubbles are gone. Tighten bleeder screw.

You should be able to crank the engine to self purge the injection lines. Make sure to tighten the hand pump back down before cranking the engine or you will get fuel leakage.
 
#8 ·
The injectors are self bleeding. The IP is also, though not the fuel filters. Make sure you are on level ground, or that you bleed the filter in the right order to get the air out.

The rise of the fuel line from the tank to the back of the engine, and subsequent drop to the IP is the main source of frustration on the SBU/MBU trucks. It makes a great opportunity for air lock as the hand primer just can't move much volume.

From the web (to save typing):

"If the pipe has high points – such as Hi1, 2 etc. and low points between them such as Lo1, 2 etc., then if the pipe is filled from the top, and was empty, the pipe fills OK as far as Hi1. If the water flow velocity is below the rising velocity of air bubbles [ref], then water trickles down to the low point Lo2 and traps the remaining air between Hi1 and Lo2. As more water flows down, the upward leg Lo2 to Hi2 fills up. This exerts a pressure on the trapped air of either H2 m of water (WG = water gauge) or H1, whichever is less. If H2 is greater than H1, then you have a full air lock, and the water level in the up leg Lo2 to Hi2 stops at H1 and no further water can flow. If H1 is greater than H2, then some water can flow, but the full pipe hydraulic head H3 will not be reached and so flow is much less than expected. If there are further undulations, then the back pressure effects add together. Obviously long pipelines over fairly level, but undulating land, are bound to have many such high and low points. To avoid air or gas lock, automatic vents are fitted which let air or gas out when above a certain pressure. They may also be designed to let air in under vacuum. There are many other design considerations for design of water pipeline systems e.g. [1]"

This is the #1 source of SBU/MBU fuel supply issues in my experience and the restriction cause by the airlock may only show at higher RPM/Fuel consumption situations. You don't want to find out about it for the first time while rolling along up a hill at 90 km/h...

Bleed well.

C.
 

Attachments

#10 ·
A few days ago I carried out maintenance on my 1300L fuel water trap replacing o-rings and flexible rubber hoses leaving the whole fuel suction line empty and necessitating a lot of priming and purging air if the IP hand primer was used.
Iain had suggested earlier of using an electric fuel pump in line to purge the line of air. Not having an electric fuel pump on hand, I tried another idea.
I temporarily fitted an outboard motor fuel primer bulb onto the suction line just before the IP and in a short time had filled the line and water trap with fuel and expelled the air. I then reconnected the fuel line back to the IP and finished off using the IP hand pump briefly to purge any air back to the fuel tank.
As I did not have to bleed air from the filters the other benefit was no mess was created.
The truck started immediately and a test run showed no further purging necessary.
This primer bulb proved to be a low cost handy device that can be kept in the toolbox in order to save time and frustration if needed again at a later time.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top